This invention relates to multiplexing a plurality of various rate subchannels onto a fixed rate channel and more particularly to defining a frame structure that accomplishes this.
In order to multiplex a plurality of various rate digital subchannels onto a fixed rate digital channel, the rate of each subchannel is generally converted to a standard rate and the converted subchannels are then multiplexed using time-division multiplexing (TDM) techniques. Depending on the rates of the subchannels, such an arrangement can make inefficient use of the capacity of the high speed channel.
Framing of the high speed channel in such TDM systems, as for example, the T1 carrier system, is accomplished by one framing bit in a 193 bit frame that also includes 24 8-bit multiplexed digital channels. Framing is established and maintained by hardware recognition of a specific pattern of those framing bits in successive frames. The hardware necessary to recognize such a framing pattern is necessarily sophisticated. Furthermore, once framing is lost, several frames are required to re-lock upon the framing pattern. In addition if the data in successive frames simulates the framing pattern, improper reframing may occur.
It is desirable therefore to define a frame structure that can be used to multiplex several subchannels of varying rates such that each subchannel is assigned an integral number of bits per frame and in addition includes sufficient information in each frame to enable framing to be unambiguously and easily detected.